1. Field of the Invention
The use of calcium aluminate clinker as a refractory aggregate in shapes and castables. The present invention provides an improved refractory aggregate based on CaO.Al2O3 clinker (hereinafter referred to as “CA” clinker and designated by the following equation CnAx, wherein preferably n is an integer from about 1 to 12 and x is an integer from about 1-24) and CnAx hydrate phases and analogs or derivatives thereof including those based on barium substitution for calcium or barium additions resulting in calcium substitution after chemical reaction and/or heat treatment, all of which are referred to herein as CA clinker. Refractory materials based on aggregate compositions having these phases singularly or combined, and exclusive of C12A7 as a major phase, are provided in the present invention.
2. Description of the Background Art
Lafarge Calcium Aluminates Inc. (Maryland, USA) markets a calcium aluminate clinker as an aggregate available under their “r50” trade name. This clinker known by those skilled in the art is about 52% alumina and is dense. The presence of a relatively large amount of C12A7 causes the LaFarge Calcium Aluminates, Inc.'s calcium aluminate clinker to melt and the product is essentially fused with little or no porosity. Thus, this composition has little utility as a refractory aggregate. The present invention provides a composition comprising a CnAx clinker that lacks significant amounts of C12A7 and that has the additional benefit of the presence of porosity and usefulness as a refractory aggregate.
It is known in the refractory arts that the addition of barium-containing materials impart a penetration resistance to materials in contact with molten aluminum. In another embodiment of the present invention, compositions are provided wherein barium is substituted into the CA clinker matrix of the present invention resulting in BA6, BC2A4 and the other analogs that increase the performance of the product and resist aluminum penetration to a great degree.
A primary area of application for the compositions of the present invention is in contact with molten aluminum. Because aluminum has such a low viscosity in the molten state, it easily penetrates the refractory used to contain it. This causes the refractory to wear. In addition, penetration of the refractory causes the freeze plane to move further into the refractory lining thereby allowing molten aluminum to penetrate further. Ideal materials are those that resist aluminum penetration and are insulating enough to have the freeze plane as close as possible to the hot face or contact face of the refractory lining. Current technology relies on the addition of penetration inhibitors that work either by reducing the porosity of the refractory, which can result in moving the freeze plane further into the lining, or by changing the wetting angle of the aluminum on the refractory. Well known examples of these penetration inhibitors are fluorospar and barium sulfate, respectively. Other ways of reducing penetration known by those skilled in the art involve the use of increasingly pure alumina as an aggregate and making this material as dense as possible to reduce open porosity and hence penetration. This increases the conductivity of the material and pushes the freeze plane into the backup refractory lining in most typical configurations known by those skilled in the art.
Therefore, in spite of the background art, there remains a very real and substantial need for a refractory composition and a refractory aggregate composition having the improved calcium aluminate clinker, and a sprayable and/or castable refractory composition, for use, such as for example, but not limited to, as a refractory lining in the production of molten aluminum.